Paper is a complex, porous, fibrous material, produced by high technology manufacturing processes. These processes include the pulping operation which creates the pulp from the basic raw or recycled fibrous material, addition of components other than the pulp, then the production of paper on the papermachine. Because of the nature of paper and of the process by which it is produced, paper without local nonuniformity in its structure does not exist. This local nonuniformity of the structure of the sheet is the uneven distribution of fibers across the sheet, and is referred to as its formation.
Papermakers try to minimize the extent of this nonuniformity in order to have a good formation. As papermachines produce sheets up to about 10 meters wide in a high speed process, papermakers are concerned not only by the quality of the paper they produce, but by the consistency of this quality across the width of the machine as well as over time. In practice the quality of paper varies with time and with position across the width of the roll. Papermakers try to minimize such variability in order to achieve the objective of a constant, high quality product. They attempt to control the variables of the pulping process, any addition of chemicals to the furnish provided to the papermachine, as well as the design, operation and control of the papermachine in order to produce paper, over time and across the width of the papermachine, having minimum local nonuniformity in sheet structure, that is, of best possible formation. Quality of paper formation is the immediate and direct consequence of the furnish provided to the papermachine, and of the design, operation and control of the papermachine that produces the sheet.
The quality of formation, so determined by the many variables in papermaking, in turn affects the properties of the sheet which characterize paper quality and thereby influence its market value; optimizing paper formation is therefore central to the objective of producing high quality paper. These paper properties include strength properties, mechanical properties, printability and coatability. While it would be desirable to control the papermaking process according to direct measurement of these properties, the extent to which this procedure is possible is very limited for some properties, and impossible for other properties, because of the long measurement time required and of the complex relationship between these numerous properties and the papermaking parameters. As formation has a strong influence on paper properties, a simpler strategy available to papermakers would be to measure the quality of formation, then to control the process parameters in order to optimize the formation quality, thereby optimizing paper quality and value. Implementation of this strategy of optimizing paper quality by optimizing paper formation is limited because current commercial test instruments for formation measurement provide results which correlate poorly with paper properties. These low correlations may be the result of the current measurements not reflecting faithfully the essential characteristics of sheet formation. As paper formation is the local nonuniformity of sheet structure, measurement of the extent and nature of the nonuniformity of paper structure is now considered.
Determination of the two-dimensional map of the local value of mass per unit area of the sheet, grams/square meter, g/m.sup.2, referred to as the local grammage of the sheet, provides the basic information about sheet structure. Radiographic techniques provide such a map of local grammage but are slow and suffer from the practical disadvantages of radiographic procedures. Therefore it has long been common practice to use the alternative of light transmission technique to generate maps of local opacity of paper, a satisfactory approximation of local grammage maps. The key advantage for the use of image analysis is that with a good equipment a map of local opacity of the sheet can be determined in less than a second, very much faster than any radiographic technique. Determination of the strength and mechanical properties of paper and the quality of printing or coating on the paper may take from minutes to months, depending on the property. The potential for use of quality of formation thus determined for fast control of papermaking is therefore evident.
The next step is how to process the information contained in such maps of local nonuniformity of paper, whether determined as local grammage or local opacity. A widely practiced method is to use all the values of the local measurement contained in such a map to compute simply the coefficient of variation of the distribution of these local measurements, i.e. the standard deviation of the local measurement divided by the sheet average value of these measurements. If the measurements are of local grammage, this coefficient of variation is defined as formation number. If the measurement is of local grey level, this index is often referred to as a grey-level based formation number.
The advantage of this method of processing the information contained in the map of local nonuniformity is its simplicity, in providing a single number proportional to the total amount of local nonuniformity. Thus, the numerous commercial formation test instruments based on light transmission image analysis currently used around the world provide a single numerical value as an index of formation nonuniformity, which may be an opacity based formation number or proportional to it, or some other single number formation index. The associated disadvantage of this method of processing the data from a map of the local nonuniformity measurements is that all the information contained in the location, on the sheet, of each such local measurement is lost. Thus although current formation test instruments may start by obtaining a two-dimensional map of the location of each measurement of local opacity, most of that information is lost whenever any single number index of formation is determined, as is present practice.
Papermakers have long known that the quality of formation of paper has a big impact on paper properties and have tried correlating these properties with values of the single number index of formation provided by the commercial paper formation measurement instruments. However, these correlations have been found to be disappointingly very low. This contradiction is due to the inadequate measurements of formation provided by these instruments: a single number is necessarily blind to how the nonuniformity of formation is distributed across the sheet, but paper properties are sensitive to the way this nonuniformity is distributed.
For this reason, even decades after the use of single-index formation test instruments became standard practice in paper companies, papermakers and printers still inspect paper visually to evaluate what their formation instruments cannot tell them. With the use of test instruments giving a single number index of formation, the eye has remained the only source of the missing information as to how the local nonuniformity is distributed. Visual inspection is expressed with terms coined by papermakers such as grainy, cloudy, mottled, which evoke some of the limitless patterns for the distribution of nonuniformity across the sheet. This visual inspection is the only way papermakers currently have to evaluate the distribution across the sheet of the local nonuniformity of sheet structure, but this method provides only subjective, observer-dependant, qualitative and over-simplified information. The incentive to go from this subjective information towards quantitative representation of how the nonuniformity of structure is distributed across the sheet has led to much research concerning paper formation.
FIGS. 1a and 1b show two sheets of paper which have exactly the same coefficient of variation of local opacity, i.e. the same grey-level based formation number. It is evident that in spite of having identical values of this index of formation, the formation of these two sheets is in reality very different. The equality of values of grey-level based formation number establishes that the two sheets have the same total amount of local nonuniformity. What is apparent visually from FIGS. 1a and 1b is that this same total amount of local nonuniformity is distributed across these two sheets in two very different ways. The sheet portrayed in FIG. 1a is referred to as of grainy formation because the local nonuniformity is on a small scale, while the sheet of FIG. 1b is described as cloudy because the local nonuniformity is on a larger scale. This example illustrates the necessity of going beyond formation number to characterize the formation of a sheet of paper. FIG. 2 illustrates the steps implemented according to one prior art method which attempts to solve this problem by using Fourier analysis in order to determine how the local nonuniformity is distributed across the sheet. An image of the sheet is obtained by transmission of light, visible or not, or of a laser beam, or of other kind of radiation, to a sensor which creates the corresponding map. In the paper by Wrist, P. E. "Dynamics of sheet formation on the Fourdrinier machine", Oxford Symposium on the Formation and Structure of Paper, p. 839-888(1962), it was proposed to use have frequential analysis methods, on 1- or 2-dimensional data of local nonuniformity of grammage or grey level. In the early 1970s a FFT-based method was described by Norman and Wahren in their article "A comprehensive method for the description of mass distribution in sheets and flocculation and turbulence in suspensions", Svensk Papperstidning, 20, p. 807-818 (1972). The use of 2-dimensional data is preferable because 1-dimensional analysis loses the pronounced anisotropy of paper, crucially important for paper properties. In these studies the results have been presented variously, including in terms of frequency/amplitude, wavelength/amplitude, floc size/floc intensity, which do not provide clear information usable by papermakers for control of paper quality.
The known prior art does not provide a sufficiently accurate, reliable and useful method to analyze paper image data to obtain paper quality.